Positive displacement blowers of the backflow type, such as Roots-type blowers have long been characterized by noisy operation and, therefore, have had little successful use in many applications such as engine superchargers for passenger cars and light trucks. Noise caused by such blowers may be roughly classified into two groups: solid borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads, and fluid borne noise caused by fluid flow characteristics such as backflow or dynamic pressure pulses.
As is well known, backflow blowers are not internal compression devices, i.e., such blowers do not mechanically compress fluid while transferring the fluid from blower inlet to outlet. For example, spaces between adjacent unmeshed rotor lobes of a Roots-type blower transfers trapped volumes of relatively low pressure inlet port air to relatively high pressure air at the outlet port. Hence, the high pressure air rushes or backflows into the spaces or transfer volumes as the leading lobe of each trapped volume traverses the outlet port boundry, thereby producing dynamic pressure pulses having amplitude proportional to pressure ratio and frequency or rotor passing frequency equal to the number of rotor lobes traversing the outlet port boundries per unit time. For a two rotor Roots-type blower with three lobes per rotor, this frequency and its harmonics may be in the range of 200 to 3,000 Hz. When such a blower is operated at a pressure ratio of about 1.5, the amplitude of the outlet port pressure pulses may be in the range of two psi rms. In an analygous manner, the inlet port may also generate dynamic pressure pulses of the same frequency but with a lower amplitude in the range of 0.5 psi rms.
Pressure pulses of these frequencies and amplitudes excite supercharger system components which then act as speakers. Further, system vibrations produced by the pressure pulses are readily transmitted to vehicle chassis/body components which then also act as speakers producing objectionable noise in the vehicle passenger compartment. The problem of noise produced by the pressure pulses is particularly acute when the system includes a intercooler mounted on the chassis/body to enhance heat exchange. In such a system the supercharger may be rigidly mounted on the engine. The engine is supported on the vehicle chassis/body in known manners by shock mounts which isolate engine vibrations and allow movement of the engine relative to the chassis/body. The intercooler is soft mounted on the vehicle chassis/body to prevent or reduce vibration links between the intercooler and vehicle chassis/body. And supercharger system ducting interconnecting the engine mounted supercharger with the chassis/body mounted intercooler and the engine intake manifold with the intercooler must allow relative movement therebetween which does not adversely affect the soft mounting of the intercooler and yet be rigid enough to prevent speaker like vibration of the ducting walls.